Rubies provide a variety of bonuses, adding % Experience gain in helms, +damage in weapons, and +Strength in Armor. The exp gain bonus from a helm is a popular socket. Strength is the mainstat for Barbarians and Crusaders and those classes often stick Rubies into their armor for the boost to damage and the secondary benefit of raising their Armor.

As of D3v2 and Reaper of Souls, a Ruby is the best option for a weapon socket for any low level or poorly-geared character. In the end game more players choose Emeralds to raise Critical hit Damage, but this is only valuable to characters who have a good Critical hit Chance value. Prior to Patch 1.0.7 Rubies were much less useful in weapons, but that patch buffed their damage considerably.

These rubies have also had their bonus damage calculations changed, and will now add damage flatly to both the minimum and maximum values on weapons

For example, if you have a weapon that does 150-200 damage and you socket a Perfect Square Ruby (which adds +30 minimum damage/+30 maximum damage), your weapon will do 180-230 damage

The boosts to high end Ruby damage were enormous, and while Emeralds remain the weapon socket of choice for most characters, Rubies are useful for some classes and builds, especially those that deal damage with skill that do not or seldom proc their damage from critical hits.